Glossary

CO2 Weather - A loose term referring to the movement of air masses with high- or low-concentrations of CO2 across a continental-scale region. These atmospheric CO2 patterns move with the winds and follow "real" weather dynamics before disappearing through atmospheric mixing. See the CO2 Weather page.

Column Average - The average mole fraction of carbon dioxide in a vertical column of air starting at some point on the earth's surface and extending to the top of the atmosphere. This is a relevant quantity for comparison with satellite observations.

Data Assimilation - The process of constraining model predictions to be consistent with observations.
Models express our understanding of basic physics and chemistry, but this grounding is often not sufficient
to produce a realistic simulation. Observations inform us about the state of the carbon system at one place
and at one time, but do not by themselves tell us about other places and times. Data assimilation brings these
together to yield a simulation that is consistent both with theory and with observations.

ECMWF - The European Centre for Medium-Range Weather Forecasts. CarbonTracker uses the ECMWF operational "forecast" model for the meteorological drivers of the TM5 transport model.

MBL - Marine boundary layer. Technically, the lowest level of the atmosphere over the ocean. Practically, this is an idealized construct of air representing the well-mixed lower atmosphere away from continental sources and sinks of CO2.

MODIS - Moderate-Resolution Imaging Spectroradiometer.

Mole Fraction - The mole fraction is defined as the number of molecules of CO2 in any given air parcel divided by the total number of all molecules (except water) in that parcel. For CO2 it is usually expressed as parts per million, abbreviated as ppm. We use the mole fraction of CO2 in (dry) air because it is a conserved quantity: it does not depend on the pressure, temperature, water vapor or condensed water content, which are all highly variable. The CO2 mole fraction is much less variable, and depends only on the history of its sources and sinks, almost all of which are caused by surface processes such as photosynthesis and respiration, and the burning of coal, oil, or natural gas. It is the magnitudes of these source processes that we want to estimate.

OCO-2 - NASA CO2 observing satellite, due to be launched in July 2014.

pCO2 - The partial pressure of carbon dioxide. Contribution of CO2 to the total atmospheric pressure. Often the amount of CO2 dissolved in seawater is expressed as pCO2, since differences between atmospheric and oceanic pCO2 imply a disequilibrium that will drive a net exchange of CO2.

PgC/yr - petagram of carbon per year. A unit of emission equal to 1x1015 grams of carbon released per year, or one billion metric tons of carbon per year (GtC/yr). Technically, this is not a flux, since flux has dimensions of mass per unit area per unit time. The carbon dioxide emission is larger than the carbon emission by the ratio of molecular mass of CO2 to the atomic mass of carbon: 44/12.

ppm - The mole fraction is defined as the number of molecules of CO2 in any given air parcel divided by the total number of all molecules (except water) in that parcel. For CO2 it is usually expressed as parts per million, abbreviated as ppm. A more formal way of expressing ppm is micromoles per mole (μmol mol-1).

Q10 - Rate constant for temperature dependence of heterotrophic respiration. Microbial respiration--the breakdown of organic carbon into CO2--is often modeled as an increasing function of temperature.

Residual - Difference of simulated and measured quantities. We compute these as model-minus-observed, which casts the observation as a "control" variable. This sign convention means that positive residuals indicate the model is too high, and negative ones means the model is too low.

Rh - Heterotrophic respiration. Conversion of organic carbon compounds into CO2 by microbial processes. "Heterotrophic" refers to the fact that the agent doing this conversion is distinct from the plant which created the organic carbon compound via photosynthesis.

Selection scheme - Method by which data providers choose which parts of a measurement time series are appropriate for analysis by models. Usually this involves screening out observations with obvious contamination from local sources, often using wind direction or other criteria. These schemes are usually implemented by our data providers, who are most familiar with the conditions at their observing sites.

Sink - Any process by which carbon dioxide is removed from the atmosphere. Sinks may be natural (e.g., photosynthesis and reforestation) or due to human activities (e.g., management of forests and other carbon sequestration strategies).

Source - Any process by which carbon dioxide is added to the atmosphere. Sources may be natural (e.g., soil and plant respiration) or due to human activities (e.g., combustion of fossil fuels and deforestation).

Time Series - Observations of a quantity taken regularly over time. The most famous time series in atmospheric carbon dioxide analysis is the Keeling curve, representing CO2 concentrations at Mauna Loa, Hawaii from 1958 onwards.

TM5 - Transport Model, version 5. An off-line model of global atmospheric tracer transport. Uses meteorological driver fields from other sources to model the advection and diffusion of atmospheric gases. Supports two-way nested grids to provide higher-resolution simulations over certain areas of the globe. See http://www.phys.uu.nl/~tm5/.

TransCom - A special project of the International Geosphere-Biosphere Programme (IGBP), Global Analysis, Interpretation, and Modeling (GAIM) Project, the objective of which was to quantify and diagnose the uncertainty in inversion calculations of the global carbon budget that result from errors in simulated atmospheric transport, the choice of measured atmospheric carbon dioxide data used, and the inversion methodology employed. See http://www.purdue.edu/transcom/.

Uncertainty Estimates - CarbonTracker's fluxes are statistical quantities, which have associated "error bars". These error bars are expressed as estimates of a confidence interval. Generally, we report the standard deviation of the flux estimate as the uncertainty; this means that we believe that there is a 68% probability that the unknown, true flux lies within one standard deviation of our best guess flux.